1. Field
The disclosure relates to the preparation of a masterbatch of diene elastomer and silica comprising at least one silica, a calcium salt and a diene elastomer latex, in particular a natural rubber latex.
2. Description of Related Art
The term “masterbatch” is understood to mean an elastomer-based composite into which a filler and optionally other additives have been introduced.
The present disclosure relates to in particular to the use of such a masterbatch in the manufacture of diene rubber compositions reinforced with an inorganic filler intended for the manufacture of tires or semifinished products for tires, in particular treads of these tires.
It is known, in order to obtain optimum reinforcing properties conferred by a filler in a tire tread and thus a high wear resistance, that it is generally advisable for this filler to be present in the elastomer matrix in a final form which is both as finely divided as possible and as homogeneously distributed as possible. In point of fact, such conditions can only be achieved insofar as this filler exhibits a very good ability, on the one hand, to be incorporated in the matrix during the mixing with the elastomer and to deagglomerate and, on the other hand, to disperse homogeneously in this matrix.
In a known way, carbon black exhibits such abilities, which is generally not the case with inorganic fillers, in particular silicas. This is because, for reasons of reciprocal affinities, these inorganic filler particles have an unfortunate tendency, in the elastomer matrix, to agglomerate with one another. These interactions have the harmful consequence of limiting the dispersion of the filler and thus the reinforcing properties to a level substantially below that which it would be theoretically possible to achieve if all the bonds (inorganic filler/elastomer) capable of being created during the mixing operation were actually obtained; these interactions tend, moreover, to increase the consistency in the raw state of rubber compositions and thus to render their processability more difficult than in the presence of carbon black.
Ever since savings in fuel and the need to protect the environment have become a priority, it has proved necessary to produce tires having a reduced rolling resistance without having a disadvantageous effect on their wear resistance.
This has been made possible in particular by virtue of the use, in the treads of these tires, of novel rubber compositions reinforced with inorganic fillers, especially with specific silicas of the highly dispersible type, capable of competing, from the reinforcing viewpoint, with a conventional tire-grade carbon black, while giving these compositions a lower hysteresis, synonymous with a lower rolling resistance for the tires comprising them, and an improved grip on wet, snowy or icy ground.
Treads filled with such highly dispersible silicas (denoted “HD” or “HDS” for “highly dispersible” or “highly dispersible silica”), which can be used in tires having a low rolling resistance sometimes described as “Green Tires” for the energy saving offered to the user (“Green Tire concept”), have been extensively described. Reference will in particular be made to Patent Applications EP 501 227, EP 692 492, EP 692 493, EP 735 088, EP 767 206, EP 786 493, EP 881 252, WO99/02590, WO99/02601, WO99/02602, WO99/06480, WO00/05300 and WO00/05301.
These documents of the prior art teach the use of silicas of the HD type exhibiting a BET specific surface of between 100 and 250 m2/g. In practice, an HD silica having a high specific surface acting as reference in the field of “Green Tires” is in particular the Zeosil 1165 MP silica (BET specific surface equal to approximately 160 m2/g) sold by Rhodia. The use of this Zeosil 1165 MP silica makes it possible to obtain good compromises as regards tire performance, in particular a satisfactory wear resistance and a satisfactory rolling resistance.
The advantage of using a silica having a high specific surface lies mainly in the possibility of increasing the number of bonds of the silica with the elastomer and thus of increasing the level of reinforcement of the latter. This is why it appears advantageous to use, in rubber compositions for tire treads, silicas having a high specific surface, possibly greater than that conventionally used of the order of 160 m2/g, in order in particular to improve the wear resistance of these treads. Nevertheless, the dispersibility of the filler and the increase in its specific surface are regarded as contradictory characteristics. This is because a high specific surface supposes an increase in the interactions between filler items and thus a poor dispersion of the filler in the elastomer matrix and also a difficult processability.
Another type of solution has been envisaged which consists, in order to improve the dispersibility of the filler in the elastomer matrix, in mixing the elastomer and the filler in the “liquid” phase. To do this, recourse has been had to an elastomer in the latex form, which exists in the form of elastomer particles dispersed in water, and to an aqueous dispersion of filler, that is to say a silica dispersed in water, commonly known as silica slurry. However, bringing the elastomer latex and the slurry into contact does not make it possible (not troublesome for the position of the problem, which relates to the silica only) to have coagulation within the liquid medium, which coagulation should make it possible to obtain a solid which, after drying, results in the production of the desired masterbatch of elastomer and silica.
This is because the silica aggregates are typically hydrophilic in nature and have an affinity with the water; thus, the silica aggregates have more affinity with the water than with the elastomer particles themselves.
Different solutions have been provided to make it possible nevertheless to obtain this coagulation and a good dispersion of the filler in the elastomer matrix in the “liquid” phase by the combined use of an agent which makes it possible to increase the affinity between the elastomer and the silica, such as a coupling agent, and of an agent which makes it possible to bring about solidification, thus coagulation, known as coagulating agent.
Thus, for example, U.S. Pat. No. 5,763,388 provides for the incorporation of silica in the rubber latex by treating the silica with a coupling agent and mixing the silica, thus treated, in the presence of conventional coagulating agents.
Patent EP 1 321 488 also intends to bring an aqueous dispersion with negatively charged silica and a diene elastomer latex into contact with an emulsion comprising a polysulphide coupling agent, in the presence of a coagulating agent, such as a polyamine.
Patent EP 1 323 775 also provides bringing an aqueous dispersion comprising an inorganic filler, such as a silica, into contact with an elastomer latex in the presence of a coagulating agent which can be composed, according the list envisaged in this document, of sodium chloride, potassium chloride, salts, acids, and the like. More preferably, this method describes an additional stage which makes possible the in situ formation of the inorganic filler during the mixing between the aqueous dispersion comprising a precursor capable of forming the inorganic filler and the latex, before adding a coagulating agent.